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ISSN: 0301-4460 (print), 1464-5033 (electronic)
Ann Hum Biol, Early Online: 1–6
!2014 Informa UK Ltd. DOI: 10.3109/03014460.2014.902993
RESEARCH PAPER
Digit ratio (2D:4D) as an indicator of body size, testosterone
concentration and number of children in human males
Magdalena Klimek
1
, Andrzej Galbarczyk
1
, Ilona Nenko
1,2
, Louis Calistro Alvarado
3
, and Grazyna Jasienska
1
1
Department of Environmental Health, Faculty of Health Sciences, Jagiellonian University Medical College, Krakow, Poland,
2
Department of Animal
and Plant Sciences, University of Sheffield, Sheffield, UK, and
3
Department of Anthropology, University of New Mexico, Albuquerque, NM, USA
Abstract
Objectives: The 2nd to 4th digit ratio (2D:4D) is thought to reflect exposure to androgens during
foetal development. This study examined the relationship between low (more masculine) and
high (more feminine) 2D:4D and body size at different stages of the life course, adult
testosterone levels and number of children among males.
Methods: Five hundred and fifty-eight men from rural Poland at the Mogielica Human Ecology
Study Site participated in this study. Life history data and anthropometric measurements were
collected. Salivary morning and evening testosterone levels among 110 men from the same
population were measured.
Results: Low 2D:4D was related to higher birth weight (p¼0.04), higher birth length (p¼0.01),
higher body mass during childhood and adolescence (p¼0.01), higher BMI (borderline
significance, p¼0.06), higher number of children among fathers (p¼0.04) and higher
testosterone levels during adulthood (p¼0.04).
Conclusions: This study shows, for the first time in a single population, that digit ratio is related
to sub-adult body size at different stages of the life course, adult testosterone levels and
number of children. The observed results suggest that digit ratio might be a valuable predictor
of male body size and reproductive characteristics.
Keywords
2D:4D, body size, number of children, rural
population, testosterone
History
Received 23 October 2012
Revised 27 January 2014
Accepted 16 February 2014
Published online 28 April 2014
Introduction
Pre-natal testosterone concentration may play a significant
role in programming the future biological condition of males,
including body size and reproductive success (Fink et al.,
2003; Manning & Fink, 2008). A number of researchers have
suggested that 2nd to 4th digit ratio (2D:4D) might be a
marker of pre-natal androgens concentration in the womb
(i.e. Manning & Bundred, 2000). 2D:4D is defined as a
proportion between the length of the index and ring fingers
and is determined during early foetal development, around
the 13th week of gestation (Manning et al., 1998). Higher
concentration of pre-natal testosterone (in relation to oestro-
gen) may be indicated by longer ring finger than index finger
and is defined as low digit ratio. In contrast, high 2D:4D
potentially indicates a greater early oestrogen exposure
(in relation to testosterone) (Manning & Bundred, 2000).
The relationship between pre-natal sex hormone concentra-
tion and finger lengths was supported by a study that shown
that males with congenital adrenal hyperplasia (CAH) – a
disease related to high pre-natal levels of androgens – had
lower values of 2D:4D (Brown et al., 2002), similarly to
males with 21-hydroxylase deficiency (responsible for
developing CAH) (O
¨kten et al., 2002). It is also hypothesized
that 2D:4D is genetically determined due to the presence of
variant rs314277 located within intron 2 of the LIN28B
gene (Medland et al., 2010) and expression of Hoxa or Hoxd
genes. These Hox genes are responsible for differentiation
of both the digits and the urinogenital system simultaneously
(Manning et al., 2003), although this hypothesis still requires
confirmation (Medland et al., 2010).
Digit ratio (as a possible marker of pre-natal sex hormone
levels) has been reported to correlate with male birth size
(Danborno et al., 2010; McIntyre et al., 2006; Ronalds et al.,
2002), adult body size (Fink et al., 2003; Van Dongen, 2009)
and family size (Manning et al., 2003). However, some other
studies have not confirmed these relationships (for a review
see: Putz et al., 2004). Additionally, there are some sugges-
tions that digit ratio predicts adult testosterone concentrations
(Garcia-Cruz et al., 2012), including patients of infertility
clinics (Manning et al., 2004), but this association was
questioned by meta-analytic review (Honekopp et al., 2007),
which did not prove a statistically significant relationship
between 2D:4D and adult circulating sex hormone levels.
Testosterone is related to body size and body composition
in males. The impact of pre-natal and post-natal testosterone
concentration on body composition may be 2-fold. High
testosterone concentration is associated with higher lean body
mass (Schroeder et al., 2012), while lower testosterone levels
are related to increased fat mass (Vermeulen et al., 1999).
Correspondence: Magdalena Klimek, Institute of Public Health,
Jagiellonian University Medical College, Grzegorzecka 20, 31–531
Krakow, Poland. Tel: +48 12 43 32 842. Fax: +48 12 421 7447. E-mail:
magdalenaannaklimek@gmail.com
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This is a possible consequence of the multifunctional effects
of testosterone (i.e. induction of muscular hypertrophy,
increased lipid uptake and higher fatty acid mobilization
from adipose tissue) (i.e. Vermeulen et al., 1999). It should
also be noted that high testosterone levels may accelerate
foetal growth and, thus, result in higher birth weight and
length, but the positive role of oestrogen also should be taken
into account (Danborno et al., 2010; Frank, 2003). Moreover,
a relationship between 2D:4D and birth size can also be
expected, since it is hypothesized that both these traits are
pre-natally programmed (Voracek, 2009). 2D:4D may also
predict family size (i.e. number of children) since it is related
to mating and reproductive preferences and sexual behaviour
(i.e. Manning & Fink, 2008), although not all studies
have confirmed such associations (Manning et al., 2000;
Sorokowski et al., 2012).
In this study we tested relationships between 2D:4D and
birth size, body size during childhood and adulthood, number
of children and testosterone concentration among men from
a traditional, rural population from Southern Poland. We
hypothesized that more masculine digit ratio (low 2D:4D),
indicating higher exposure to pre-natal androgens, will predict
larger body size, higher testosterone levels and a higher
number of children.
Materials and methods
The study was conducted in a village with a high birth rate,
which is a part of the Mogielica Human Ecology Study Site,
located in Southern Poland (for description of the study site
see Jasienska & Ellison (2004) and Jasienska (2013)). Two
groups of men were studied. Group 1 consisted of 558 males
who participated in the study between years 2003–2009.
This group was divided into two sub-groups: Group 1A:
Children and adolescents (3–22 years old) and Group 1B:
Fathers (23–89 years old). Data on demographic, anthropo-
metric and family characteristics were collected. Participants’
birth weight and length were obtained from personal health
records. Body weight, body height, right and left hand
finger digits were measured. Body weight, body height and
BMI for each child and adolescent were individually
compared to WHO Child Growth Standards (2006) and
calculated as a z-score. Finger lengths were measured directly
on the ventral surface of the palm by a trained assistant,
using a manual calliper. The measurements of second and
fourth finger length in both hands were taken from proximal
finger crease to the distal tip of the finger, according to a
previously published procedure (i.e. Manning et al., 1998), but
to the nearest 0.1 cm. Participants with arthritis or finger
injury (i.e. due to manual labour) were excluded from the
study.
Group 2: Testosterone Study consisted of 110 men
surveyed in 2011, aged 18–78 (mean ¼39.5, SD ¼17.82)
from the same village. Saliva samples for testosterone
assessment were self-collected by participants in polypropyl-
ene tubes and frozen within 8 hours of collection. Each
participant took two saliva samples: in the morning, shortly
after waking up and at night, just before going to bed. Saliva
was analysed for testosterone at the Hominoid Reproductive
Ecology Laboratory, University of New Mexico, using an
enzyme immunoassay kit manufactured by Salimetrics
(State College, PA; Kit No. 1-2402).
Statistical analysis
Participants from both groups (1 and 2) were further divided
into two groups based on digit ratio value: low 2D:4D
(2D:4D51) or high 2D:4D (2D:4D 1), following previously
published studies (i.e. Bang et al., 2005; Seo et al., 2010).
Differences between groups with low and high 2D:4D in birth
weight, birth length, body weight, body height, BMI during
childhood and adolescence and testosterone concentration
were tested by Student’s ttest or Mann–Whitney Utest
(depending on whether a particular variable had a normal
distribution or not). Differences in body weight, body height,
BMI and the number of children in adulthood were tested by
analysis of covariance (ANCOVA), with age as a potential
confounder. Additionally, all analyses were repeated with
right- and left-hand 2D:4D as a continuous variable in
regression analysis. Simple or multiple (including partici-
pant’s age) regression analyses were performed. Statistical
analyses were conducted in Statistica package version 9.0.
An alpha of 0.05 was set to determine statistical significance.
Results
Means (SD) of 2D:4D in both hands, age and anthropometric
measures of studied groups (Group 1A, Group 1B and
Group 2) are presented in Table 1 and means (SD) of 2D:4D
in groups with high and low digit ratio in both hands are
presented in Table 2. Groups of men with low and high digit
ratio differed in size at birth and childhood body size, number
of children and mean testosterone levels (Table 3). Among
children and adolescents, a group with low right-hand 2D:4D
had higher birth weight (p¼0.04) and higher birth length
(p¼0.01). In this group, boys with low right-hand 2D:4D had
higher body mass (when compared to norms published by the
World Health Organization (2006) and calculated as z-scores)
Table 1. Characteristics of study groups (mean and standard deviation).
Mean SD
Group 1A: Children and adolescents, n¼320
Right hand 2D:4D 0.97 0.05
Left hand 2D:4D 0.97 0.05
Age (years) 10.8 6.69
Birth weight (g) 3423.6 539.21
Birth length (cm) 55.4 3.21
Body mass (z-score) 0.43 1.177
Body height (z-score) 0.16 1.091
BMI (z-score) 0.23 1.136
Group 1B: Fathers, n¼238
Right hand 2D:4D 0.97 0.05
Left hand 2D:4D 0.97 0.05
Age (years) 51.9 15.92
Body mass (kg) 78.6 14.19
Body height (cm) 171.1 6.74
BMI (kg/m
2
) 23.4 4.22
Number of children 3.7 2.27
Group 2: Testosterone Study, n¼110
Right hand 2D:4D 0.97 0.04
Left hand 2D:4D 0.97 0.04
Age (years) 39.5 17.82
Testosterone levels (pmol/L) 270.3 100.22
2M. Klimek et al. Ann Hum Biol, Early Online: 1–6
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than boys with high 2D:4D (p¼0.01). We observed a
difference in BMI (borderline significance, p¼0.06) – boys
with low right-hand 2D:4D had higher BMI than boys with
high 2D:4D, when also calculated as z-scores. In addition,
we found no difference between groups of children and
adolescents with low and high right-hand 2D:4D in body
height (p¼0.36).
Fathers had between 1–12 children (mean ¼3.69,
SD ¼2.27). Fathers with low right-hand 2D:4D had a
higher number of children relative to the high 2D:4D group,
after controlling for age (p¼0.04, Table 3). No statistically
significant differences were observed among fathers in body
mass (p¼0.36), body height (p¼0.34) or BMI (p¼0.36).
Furthermore, among participants from Group 2: Testosterone
Study, mean testosterone concentration (calculated as a mean
of morning and evening values) was 270.6 [pmol/L]
(SD ¼100.22). Men with low right-hand digit ratio had
higher testosterone concentrations than those with high right-
hand 2D:4D (p¼0.04) and these participants did not differ in
mean age (p¼0.1, for both hands). All tested differences
between groups with low and high digit ratio were not
statistically significant for the left hand (Table 3).
When we re-ran all analyses with 2D:4D as a continuous
variable in simple or multiple regression models some of the
results remained statistically significant. We observed that
right hand 2D:4D is negatively related to body mass
(r¼0.18, p¼0.01) and to BMI during childhood and
adolescence (r¼0.14, p¼0.02), when body weight and
BMI were calculated as z-scores. Additionally, we observed
results with borderline statistical significance: negative rela-
tionship between left-hand digit ratio and birth weight
(r¼0.08, p¼0.08), right-hand 2D:4D and birth length
(r¼0.11, p¼0.08) and left-hand digit ratio and number
of children among fathers (r¼0.09, p¼0.09), when
participants age was included to the model.
Discussion
This study, for the first time, documents that men’s digit ratio
(a putative measure of prenatal androgen concentration) is
associated with several life history traits in a single popula-
tion. We show that low right-hand 2D:4D was related to
larger size at birth, higher body mass during childhood and
adolescence, higher testosterone levels and higher number
of children. This indicates a possible long-term effect of
pre-natal androgens on male life history. However, some
of the results obtained in group comparisons were not
confirmed when regression models were used. The statistical
approach of group comparison was chosen to compare
participants with low more masculine and high more
feminine 2D:4D. Since body size, testosterone levels and
fertility are influenced by many factors we did not expect to
find strong, linear correlations with 2D:4D. We rather
hypothesized that there are differences between groups of
men who are more or less ‘‘masculine’’ (based on their foetal
hormonal exposure).
Our results, based on analysing 2D:4D in groups with high
and low digit ratio, show that a low digit ratio is related to a
larger birth size in males. A similar relationship was shown
in a sample of British men among whom low 2D:4D was
related to higher birth length (Ronalds et al., 2002) and in a
sample of Nigerian men where low 2D:4D was related to
higher birth weight (Danborno et al., 2010). This may be due
to a role of testosterone in determining body composition,
especially its positive impact on somatic growth (Danborno
et al., 2010) and development of lean body mass (Schroeder
et al., 2012). Higher body mass and higher BMI (calculated
as z-score) among children and adolescents with low 2D:4D
(borderline significance for BMI) was observed. However,
adult body size was no longer predicted by 2D:4D. Results
of previous studies investigating the relationship between
adult body size and 2D:4D were inconclusive. For example,
Barut et al. (2008) and Danborno et al. (2008) did not observe
statistically significant relationships, while other studies
documented either a positive relationship between 2D:4D
and adult body size—heavier males tended to have a lower
digit ratio (i.e. Van Dongen, 2009; Almasry et al., 2011) or a
negative impact of pre-natal testosterone on body mass—
heavier males had a higher digit ratio (i.e. Fink et al., 2003).
Such discrepancies between our results and some of the
previous findings may occur because of inter-population
variation in nutritional and behavioural factors that influences
body composition and size throughout the lifespan. Moreover,
pre-natal testosterone concentration is more likely to influ-
ence birth size and body size in childhood (due to the
shorter period of time elapsing since pre-natal exposure) than
body size in adulthood, when many different factors have
been influencing growth. This is exactly the pattern that we
observed in the studied population. It is also likely that in
adults other traits are better markers of adult body compos-
ition than body weight or BMI. For example, 2D:4D
correlated with waist-to-hip ratio (Manning, 2002) and hand
grip strength (a proxy of muscular strength) (Fink et al., 2006;
Hone & McCullough 2012; Zhao et al., 2012) among males
and with waist-to-chest ratio (Fink et al., 2003) among
females. Analyses of the 2D:4D relationship and body
composition using different pointers should be taken into
account in future studies.
Our study also shows that men with low 2D:4D have a
higher number of children. Similar findings were reported
in other studies assessing the relation between digit ratio and
males’ fertility (Manning et al., 2000; Manning & Fink, 2008;
Voracek et al., 2010). Despite the fact that there are many
cultural and behavioural factors which may affect males’
Table 2. Characteristics of low and high 2D:4D (mean and standard deviation) among study groups.
Low right 2D:4D High right 2D:4D Low left 2D:4D High left 2D:4D
Mean (SD) Mean (SD) Mean (SD) Mean (SD)
Group 1A: Children and adolescents 0.95 (0.36) 1.02 (0.03) 0.95 (0.03) 1.02 (0.03)
Group 1B: Fathers 0.95 (0.04) 1.03 (0.04) 0.94 (0.04) 1.03 (0.05)
Group 2: Testosterone Study 0.95 (0.03) 1.01 (0.02) 0.95 (0.03) 1.01 (0.01)
DOI: 10.3109/03014460.2014.902993 Digit ratio (2D:4D) and body size and reproduction in males 3
Ann Hum Biol Downloaded from informahealthcare.com by 84.10.160.45 on 04/28/14
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reproductive success, pre-natal hormonal environment
(reflected in 2D:4D value) seems to play a role as well.
Variables associated with male fertility such as sperm
number (Manning et al., 1998), sex drive, level of sexual
excitement (Manning & Fink, 2008), number of sexual
partners per individual (Honekopp et al., 2006) and age at
first marriage (Sorokowski et al., 2012) were all related to
2D:4D. These results might suggest that digit ratio might
be widely associated with traits related to male reproductive
strategy.
We also show that pre-natal testosterone levels (reflected
in 2D:4D values) predict adult testosterone concentration.
In previous studies, in males referred for prostate biopsy,
those with high 2D:4D had lower testosterone serum levels
(Garcia-Cruz et al., 2012) and patients attending infertility
clinics tended to have 2D:4D (Manning et al., 2004).
However, the majority of other studies did not detect
statistically significant relationships between 2D:4D and
testosterone levels (i.e. Bang et al. 2005; Honekopp et al.,
2007; Muller et al., 2011). We previously demonstrated a
relationship between pre-natal characteristics and adult sex-
hormone levels in Polish women (Jasienska et al., 2006a,b)
and results of the present study suggest that pre-natal
environment may influence levels of male sex hormones
as well.
It is also worth mentioning that all our results are
significant only for right-hand digit ratio. It is likely that
pre-natal testosterone (early developmental masculinization)
influences a delay of development of left side of the
body traits (Geschwind & Galaburda, 1985) and may also
lead to stronger expression of sexually dimorphic traits
on the right side of the body (Tanner, 1990). Furthermore,
a meta-analysis of 116 studies suggested that right-hand
digit ratio might be a more suitable marker of pre-natal
sex hormones exposure than left-hand (Honekopp &
Watson, 2010).
Several limitations of our study should be taken
into account. First, our results should be replicated in a
study in which assessment of adult testosterone levels would
be based on a higher number of samples for each participant,
since testosterone levels can be influenced by many physio-
logical and behavioural factors, i.e. stress level, sexual
relationships, parenting or male competition (Gray &
Campbell, 2009; Jasienska et al., 2012) and thus frequent
sampling would lead to more reliable results. Second, due
to the study protocol and long-term duration of the study,
finger measurements were conducted only once for each
participant and participants taking part in a study in different
years were measured by different study assistants. However,
we did not observe any differences in mean finger lengths
among years of study for: right-hand index finger
F
6,539
¼0.28, p¼0.95, right-hand ring finger F
6,534
¼0.20,
p¼0.97, left-hand index finger F
6,540
¼0.13, p¼0.99 and
left-hand ring finger F
6,542
¼0.10, p¼0.99 in ANOVA
analyses. This suggests high reliability of measurements.
Finally, further studies are needed in order to determine at
what age effects of pre-natal environment on body size are
no longer significant. It can be hypothesized that this
occurs around puberty, but our sample size did not allow
for testing of this hypothesis.
Table 3. Differences between groups with low and high, right- and left-hand 2D:4D in birth size, body size in childhood and adulthood, testosterone levels and number of children. pValue was derived from
Student’s ttest, Mann–Whitney Utest or analysis of covariance.
Right 2D:4D Left 2D:4D
Low 2D:4D High 2D:4D Low 2D:4D High 2D:4D
Mean (SD) Mean (SD) t/F
a
df pMean (SD) Mean (SD) t/F
a
df p
Group 1A: Children and adolescents n¼179 n¼113 n¼196 n¼128
Birth weight (g) 3482 (469) 3352 (599) 1.98 267 0.04 3369 (505) 3358 (583) 1.7 297 0.09
Birth length (cm) 55.87 (2.95) 54.88 (3.58) 2.47 250 0.01 55.60 (2.94) 55.22 (3.59) 0.91 275 0.3
Body mass in childhood and adolescence z-score
b
0.60 (1.17) 0.14 (1.15) 2.56 178 0.01 0.47 (1.20) 0.35 (1.17) 0.70 178 0.4
Body height in childhood and adolescence z-score
b
0.19 (1.07) 0.08 (0.98) 0.92 289 0.3 0.14 (1.07) 0.17 (0.99) 0.26 288 0.7
BMI in childhood and adolescence z-score
b
0.33 (1.28) 0.08 (1.06) 1.84 287 0.06 0.26 (1.19) 0.21 (1.08) 0.40 286 0.7
Group 1B: Fathers n¼129 n¼74 n¼134 n¼76
Body mass in adulthood (kg) 78.39 (13.54) 80.16 (15.54) 0.85 199 0.3 77.46 (15.30) 81.00 (12.31) 3.13 207 0.07
Body height in adulthood (cm) 171.44 (6.47) 170.60 (7.35) 0.90 201 0.3 170.92 (7.04) 171.24 (6.28) 0.27 208 0.6
BMI in adulthood (kg/m
2
) 23.33 (3.90) 23.86 (4.62) 0.85 199 0.3 23.05 (4.55) 24.10 (3.66) 3.13 207 0.08
Number of children 3.8 (2.42) 3.2 (2.01) 4.45 200 0.04 3.8 (2.21) 3.4 (2.38) 2.15 207 0.1
Group 2: Testosterone study n¼77 n¼31 n¼77 n¼33
Testosterone levels (pmol/L) 283.17 (106) 239.9 (89.4) 1.99 106 0.04 275.77 (91.95) 258.50 (123) 0.81 48 0.4
tcoefficient corresponds with student’s ttest or Mann–Whitney Utest. Fcoefficient corresponds with analyses of covariance.
a
Depending on the type of analysis.
b
Calculated according to World Health Organization norms.
4M. Klimek et al. Ann Hum Biol, Early Online: 1–6
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Conclusion
In conclusion, 2nd to 4th digit ratio, a potential biomarker of
pre-natal testosterone exposure, is related to birth size and
subsequent sub-adult body size, adult testosterone levels and
number of children among men. This indicates a long-term
effect of pre-natal androgens on male life history.
Acknowledgements
We are grateful to Michal Jasienski, Ludwik Odrzywołek, Reverend Jan
Gniewek, Reverend Stanisław Krzywonos (Słopnice Parish, Poland),
Janusz Dziedzic, MD, research assistants and to our study participants.
We also thank Melissa Emery Thompson and the Hominoid
Reproductive Ecology Laboratory, University of New Mexico, for
assistance with hormone analysis. We are grateful to two anonymous
reviewers for comments that helped to improve the manuscript.
Declaration of interest
The authors report no conflicts of interest. The authors alone are
responsible for the content and writing of the paper.
This study was supported by grants from the National Science Centre
(grant no. N N404 273440) and Ministry of Science and Higher
Education (grant no. IdP2011 000161), Salus Publica Foundation the
Foundation for Polish Science (I.N.) and Yale University Program in
Reproductive Ecology.
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